MIRI Imaging Recommended Strategies

This page gives  recommendations that, together with the MIRI Generic Recommended Strategies and the specific TSO recommendations, should help the observer to plan MIRI Imager observationsNote that these are pre-launch recommendations (as of November 2017) that will be updated with results from on-orbit commissioning.

On this page

The MIRI Imager offers 9 broadband filters covering wavelengths from 5.6 to 25.5 μm (Bouchet et al. 2015). Observers should follow the mode-independent general guidelines described for MIRI. This page focuses on aspects that are relevant for most imager science observations. Specific MIRI imaging time series observations recommendations can be found here. 



Detector readout mode

See also: Understanding Exposure TimesMIRI Generic Recommended Strategies (Detectors)

The default readout mode for imaging is FAST, but proposers may want to consider other strategies.  FAST mode is mandatory for subarray exposures, whereas FULL array observations may use SLOW mode.



Dithering

See also: MIRI Imaging Dithering

For most science cases ditheringis a highly recommended and necessary practice for the following reasons:

  • Allows good PSF sampling. This is mostly relevant when using the F560W filter; the MIRI imager Nyquist samples the PSF for wavelengths ≥ 6.25 μm.
  • Minimizes detector cosmetics and defects. 
  • Makes possible accurate background measurements for point sources, and at longer wavelengths permits tracking of potential telescope thermal emission variations.
  • Mitigates the impact of bad pixels.
  • Allows tracking detector drifts at the timescale of the dwell time per dither position (i.e. total length of time the telescope exposes in a dither position).

The Astronomer's Proposal Tool (APT) offers a set of pre-defined dither patterns for imaging.  Photometric time-series observations may make use of a no-dither option.  There are two main challenges to dithering: (1) choosing an adequate pattern and (2) deciding the dwell time (i.e., for how long to stay integrating in a single dither position). 

Choosing a dither pattern  

The user has to select a dither pattern that ensures enough redundancy, hence good quality, in the data. Below is the list of MIRI imaging dithers offered by the APT with usage recommendations.


Table 1. MIRI imaging dithers offered by the Astronomer's Proposal Tool

Dither patternApplicabilityCharacteristicsUsage
4-Point-SetsBroad

Choices of point and extended sources. Good PSF sampling in all subarrays/bands.

Allows offsets to accommodate multiple exposures.

Optimized dither solution for most science cases.
CYCLINGBroadGaussian-distributed cloud of points.Good dither solution for many science cases. The user has to define the pattern by choosing the points within a cloud.
2-PointLimited2 separated exposures. Allows for simple background subtraction.

The use of this pattern will have to be justified, both in terms of science use and data quality.

Use example: shallow mosaic where each tile has larger separation than 20”. Each mosaic position could perform a 2 pt dither, ensuring both good redundancy and larger coverage.

REULEAUXSpecialized12 points dither in different sizes to fit subarrays.Spitzer users may be familiar with this pattern. Categorized as specialized because it is complex to design observations that are optimized for its use.


Dwell time limit

Dwell time defines how long you can stay at a single dither position.  Since multiple exposures are not allowed a single dither position, this time also defines your exposure length.  The following table gives recommendations on the length of time observers should spend in a single dither position (i.e., exposure length). These estimations are based on ground measurements of flight-like detectors. 


Table 2. Recommendations on the length of time at a single dither position (exposure length)

MIRI filterBackground typeLimitation in dwell time?Recommendation
F560W to F1800WLowNo

If possible, a minimum of at least 40 groups in FULL/FAST mode (111 seconds)  will minimize the effects of drifts at low backgrounds.  A maximum of 360 groups in FULL/FAST is recommended (although not required).

F560W to F1800WMediumNoA minimum of 5 groups and a maximum of 1000 seconds integration is recommended  (but not mandatory).
F2100W and F2550WAlways highYes

Maximum dwell time of 8 minutes; longer exposures will reach a ceiling in the SNR.



Target acquisition

See Main Article: MIRI Generic Recommended Strategies (Target Acq)

Target Acquisition (TA) is currently not being offered for the MIRI imager. Given that the observatory absolute pointing accuracy is expected to be about 0.45" (1-σ radial error) for the MIRI MRS, and slightly better for the imager, most imager science use cases will not need Target Acquisition. However, users interested in imaging TSO science may wish to have TA capabilities for at least the SUB64 subarray.



Filter ordering

At longer wavelengths, the MIRI imager data will be affected by an additional high background component coming from the telescope emission that can potentially imprint latents in the detector. To avoid persistence due to transitions from high to low backgrounds, it is best to sort the imager exposures from short to long wavelengths. The observer has control on the order in which the filters are used: it is the same one that is specified in the MIRI Imaging Template APT at the time of submitting the proposal.



Background observations

See also: JWST Background Model, Background-Limited JWST ObservationsMIRI Generic Recommended Strategies (Background)

In addition to general background information for MIRI, imaging observations should consider:

  • Point/isolated sources will already provide information about the JWST background and no additional background exposure will be needed.
  • Extended sources should have an associated background observation for any observing period. Background information could be also obtained in a mosaic.

When an observation program assigns a background to a science target, that creates a formal association between them. By doing this, the pipeline will automatically subtract the background exposure from the target exposure. To avoid having undesired residuals from this step, the user should:

  • Choose a background area that is as clean as possible of sources. Given the MIRI sensitivity, it is unlikely to find "empty" regions in the imager FOV (about 74" × 113" in FULL array).
  • Dither the background target. By doing that the pipeline will stack the dithered images thus removing unexpected sources, and use that combination to remove the background from the science data. Observers should carefully consider how many dither points will be needed to remove sources present in the background region with the stacking technique.


References

Bouchet et al., 2015, PASP, 127, 612B
The Mid-Infrared Instrument for the James Webb Space Telescope, III: MIRIM, The MIRI Imager

Glasse et al., 2015, PASP, 127, 686G
The Mid-Infrared Instrument for the James Webb SpaceTelescope, IX: Predicted Sensitivity




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